Display device and driving method thereof

ABSTRACT

A liquid crystal display is suitable for displaying images with rapid motions, and comprises an active matrix substrate equipped with a plurality of thin film transistors. The active matrix substrate comprises a plurality of pixels that are placed at the encircled areas of a plurality of scanning lines and a plurality of data lines. Each pixel consists of two thin film transistors and one pixel electrode. The data lines connected electrodes of the thin film transistors are connected to two adjoining data lines respectively, whereas the pixel connected electrodes of the two thin film transistors are together connected to the pixel electrode. The gate electrodes of the two thin film transistors are connected to two adjoining scanning lines respectively.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a display, such as a liquid crystaldisplay (LCD) panel and more particularly to a display device and adriving method thereof that speeds up optical response and is suitablefor displaying continuous images with rapid motions.

2. Description of the Related Art

Progress has been ceaselessly made in the manufacturing technique forliquid crystal displays regarding high contrast displays with a wideview angle. However, continuous images with rapid motions are displayedat the expense of blurred images, as an image change takes place laterthan variations in motions do. So far various related driving techniqueshave been put forth in an attempt to shorten the time liquid crystaldisplays take to respond. Of these, the capacitively coupled driving(CCD) method puts forth by the Japanese Matsushita Electric IndustrialCo., Ltd. shortens the time pixel electrodes take to respond tovariations in voltage best, and in consequence it speeds up changes inthe electric field of a liquid crystal capacitor.

FIG. 1 is an equivalent circuit diagram of a conventional liquid crystaldisplay. The liquid crystal display 10 is composed of a plurality ofparallel data lines 121-12 n and a plurality of scanning lines 111-11 mdisposed perpendicular to the data lines, which further comprises aplurality of pixels 13 are placed at the encircled areas of the datalines 121-12 n and the scanning lines 111-11 m. Each pixel 13 comprisesa thin film transistor 131 and a liquid crystal capacitor 133, whichcontrol the direction in which liquid crystal molecules tilt. Forinstance, the thin film transistor 131 is controlled by a pulse Φ₂ ofthe scanning line 112 to be turned on or turned off, whereas the twoelectrodes of the liquid crystal capacitor 133 are connected to a pixelelectrode 134 and a common electrode 135, respectively. In addition,each of the pixels 13 comprises a storage capacitor 132 whose twoelectrodes are connected to the pixel electrode 134 and the scanning111, respectively. With the storage capacitor 132, the operating voltageof the pixel electrode 134 is kept within a preferred voltage range soas to reduce the leakage resulting from the properties of liquid crystalmaterials and other stray capacitance.

Furthermore, a coupled voltage is induced on the pixel electrode 134 byemploying the scanning signals Φ₁-Φ_(m), with four gate voltage levels.Given the coupled voltage, the electric field of the liquid crystalcapacitor 133 varies faster. However, real changes of gray levels alwaysoccur later than variations in the electric field, as liquid crystalmolecules tilt slower than the electric field varies.

FIG. 2( a) is a conventional waveform diagram of the optical responsesof pixels and data signals. The waveforms of the driving voltage appliedto a pixel electrode are shown in the lower half of the figure. In theupper half of the figure, the dotted line indicates the theoreticaloptical responses of pixels, whereas the solid line indicates the actualoptical responses of pixels. If a second default voltage V₁ applied tothe pixel electrode changes into a first default voltage V₂, the actualpositions or states of the liquid crystal molecules vary and thus thetransmittance of rays from a backlight source decreases, compared withthe ideal state, which is a true white display without any appliedvoltage. It takes about two vertical scanning periods to pass through atransient time where transmittance decreases from L₂ to L₁, thus theconventional technology is unfit for displaying continuous images withrapid motions.

The concept of fast response driving is put forth to speed up opticalresponses, as shown in FIG. 2( b). The default voltages V₂ and V₁applied to the pixel electrode are replaced with V₂′ and V₁′, whereV₂<V₂′ and |V₁|<|V₁′|. Hence, the time taken to pass through a transienttime in which transmittance decreases from L₂ to L₁ can be reduced toapproximately one vertical scanning period, indicating that the fastresponse driving method surpasses the driving method described in FIG.2( a) in displaying continuous images with rapid motions. Furthermore,the deviation area A′ (that is, the hatched area enclosed by the solidline and the dotted line) of the fast response driving method is lessthan the hatched area A in FIG. 2( a), thus the fast response drivingmethod seldom brings about blurred motion images.

FIG. 2( c) is a diagram about the waveforms of the optical responsesresulting from the Dynamic Contrast Compensating Driving method putforth by Japanese Hitachi Ltd. Wherein the driving voltages V₂″>V₂ and|V₁″|>|V₁|, an actual optical response ends up with an overshootingwaveform during a vertical scanning period and results in the return ofthe default transmittance L₂ or L₁ during the following period. Theovershooting-related area B is roughly equal to the area A″ so as tocompensate for a lack of dynamic contrast in motion images.Nevertheless, it still takes longer time than one vertical scanningperiod to bring about an overshooting waveform with the dynamic contrastcompensating driving method, making it impossible to apply the dynamiccontrast compensating driving method to motion images, which take lessthan 16.7 ms to give an optical response.

SUMMARY OF THE INVENTION

The first objective of the present invention is to provide a liquidcrystal display and a driving method for thereof capable of giving fastoptical responses, wherein two thin film transistors together functionas the switches of each pixel; one of the thin film transistors allowsnormal driving voltage to a pixel electrode; meanwhile, the other thinfilm transistor allows overdriving voltage to be applied to the pixelelectrode; as a result, the liquid crystal display gives opticalresponses much sooner.

The second objective of the present invention is to provide a liquidcrystal display compatible with existing driving devices, so that thecircuit of the liquid crystal display works without any newly developeddriving devices.

In order to achieve the aforesaid objectives, the present inventiondiscloses a liquid crystal display that is suitable for displayingimages with rapid motions and comprises an active matrix substrateequipped with a plurality of thin film transistors. The active matrixsubstrate comprises a plurality of pixels that are placed at theencircled areas of a plurality of scanning lines and a plurality of datalines. Each pixel consists of two thin film transistors and one pixelelectrode.

The data lines connected electrodes of the thin film transistors areconnected to two adjoining data lines respectively, whereas the pixelconnected electrodes of the two thin film transistors are togetherconnected to the pixel electrode. The gate electrodes of the two thinfilm transistors are connected to two adjoining scanning linesrespectively.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described according to the appended drawings inwhich:

FIG. 1 is an equivalent circuit diagram of a conventional liquid crystaldisplay;

FIGS. 2( a)-2(c) are waveform diagrams of the optical responses ofpixels and data signals in accordance with prior arts;

FIG. 3 is a schematic diagram of the circuit layout for the activematrix substrate of the liquid crystal display in accordance with thepresent invention;

FIG. 4 is a magnification of the circuit layout of portion C in FIG. 3;

FIG. 5 is a waveform diagram of the optical response of a pixel and adata signal in accordance with the present invention;

FIG. 6 is a waveform diagram of the optical responses during severalconsecutive vertical scanning periods in accordance with the presentinvention;

FIG. 7 is a schematic diagram of the circuit layout of the pixel appliedto an active matrix substrate employing the color filter on arraytechnology in accordance with the present invention; and

FIG. 8 is a schematic diagram of the circuit layout of the pixel appliedto an active matrix substrate employing the Advanced-Super IPStechnology in accordance with the present invention.

PREFERRED EMBODIMENT OF THE PRESENT INVENTION

FIG. 3 is a schematic diagram of the circuit layout for the activematrix substrate of the liquid crystal display in accordance with thepresent invention. The active matrix substrate 30 comprises not only afirst data driving device 341 that drives first data lines 321, 323 and325, but also a second data driving device 342 that controls second datalines 322 and 324. The first data driving device 341 and the second datadriving element 342 can be disposed on the two opposite sides of theactive area D where the pixels exist. Similarly, a first gate drivingdevice 351 controls first scanning lines 311, 313 and 315, whereas asecond gate driving device 352 controls second scanning lines 310, 312and 314. The first gate driving device 351 and the second gate drivingdevice 352 can be disposed on the two opposite sides of the active areaD respectively.

FIG. 4 is a magnification of the circuit layout of portion C in FIG. 3.A pixel 33 comprises a first thin film transistor 331 and a second thinfilm transistor 332, and the two thin film transistors together functionas switches, allowing the overdriving signals on the first data line 321and the data signals on the second data line 322 to be written in aliquid crystal capacitor 333, wherein the first data line 321 isconnected to a first electrode of first thin film transistor 331 and athird electrode the second thin film transistor 332 is connected to thesecond data line 322.

The first gate electrode of the first thin film transistor 331 iscontrolled by the first scanning line 311, whereas the second gateelectrode of the second thin film transistor 332 is controlled by thesecond scanning line 312. Both the first thin film transistor 331 andthe second thin film transistor 332 are connected together to a pixelelectrode 334 electrically, wherein the pixel electrode 334 is connectedto the second electrode of the first thin film transistor 331 and thefourth electrode of the second thin film transistor 332.

Further, the pixel electrode 334 and a common electrode 335 are the twoelectrodes of the liquid crystal capacitor 333. Positions or postures ofliquid crystal molecules (not shown in the figure) in the liquid crystalcapacitor 333 change while the liquid crystal molecules are driven bythe electric field of the liquid crystal capacitor 333.

As shown in FIG. 5, since the first thin film transistor 331 is solelycontrolled by the first scanning line 311 to be turned on or turned offand a first overdriving voltage V₂* (V₂*>V₂″>V₂) or a second overdrivingvoltage V₁* (and |V₁*|>|V₁″|>|V₁|) is applied to the pixel electrode334, the optical response based on the disclosure of the presentinvention is superior to the optical response based on conventionaltechnology described in FIG. 2( c). Since the overshooting waveform ofthe transmittance always recovers the default transmittance L₁ duringthe same period, the merit of the present invention is even moredistinct.

The description of a driving method for the present invention needs torefer to FIG. 4 and FIG. 5:

The first overdriving voltage V₂* is written into the first thin filmtransistor 331 and the pixel electrode 334 via the first data line 321,and the first default voltage V₂ is written into the second thin filmtransistor 332 and the pixel electrode 334 for a start. Then the secondoverdriving voltage V₁* is written into the first thin film transistor331 and the pixel electrode 334 via the first data line 321, and thesecond default voltage V₁ is written into the second thin filmtransistor 332 and the pixel electrode 334. The value of suggestionrange between the first default voltage V₂ and the second defaultvoltage V₁ depends on the mode of liquid crystal display. For example,the value of suggestion range for TN mode is 0.5-8.5 Voltages, but forAS-IPS mode is 0.5-15.5 Voltages.

The first overdriving voltage V₂* is written during a first verticalscanning period, and a first overshooting waveform of transmittanceresulting from the first overdriving voltage V₂* returns to the firstdefault transmittance L₁ during the first vertical scanning period. Onthe other hand, the second overdriving voltage V₁* is written during asecond vertical scanning period, and a second overshooting waveform oftransmittance resulting from the second overdriving voltage V₁* returnsto the second default transmittance L₂ during the second verticalscanning period.

The difference between the first overdriving voltage V₂* and firstdefault voltage V₂ depends on the first overshooting waveform oftransmittance resulting, which further comprises a first hatched area A*and a second hatched area B* are enclosed by the first overshootingwaveform of transmittance and a first default waveform of transmittance,and the measure of the first hatched area A* is equal to the measure ofthe second hatched area B*. Further, the suggestion range of thedifference value is 0-0.5 Voltages.

The difference between the second overdriving voltage V₁* and seconddefault voltage V₁ depends on the second overshooting waveform oftransmittance resulting, which further comprises a third hatched area C*and a fourth hatched area D* are enclosed by the second overshootingwaveform of transmittance and a second default waveform oftransmittance, and the measure of the third hatched area C* is equal tothe measure of the fourth hatched area D*. Further, the suggestion rangeof the difference value is 0-0.5 Voltages.

FIG. 6 is a waveform diagram of the optical responses during severalconsecutive vertical scanning periods in accordance with the presentinvention. As illustrated with FIG. 6, in terms of optical responsespeed and dynamic contrast compensation, the present invention is moresuitable for displaying continuous images with rapid motions, as thepresent invention ensures the recovery of the default transmittanceduring every vertical scanning period and meets the dynamic contrastcompensation requirement. In addition, the circuit layout put forth inaccordance with the present invention may be applied to various types ofliquid crystal displays, such as twisted nematic (TN), super TN (STN),in-plane switching (IPS), advanced-super IPS (AS-IPS) and multi-domainvertical alignment (MVA). The present invention is also applied toliquid crystal displays characterized by a high aperture ratio (HAR)technology or a color filter on array (COA) technology.

FIG. 7 is a schematic diagram of the circuit layout of the pixel appliedto an active matrix substrate employing the color filter on arraytechnology in accordance with the present invention. In this embodiment,a first thin film transistor 331′ and a second thin film transistor 332′are disposed at two diagonal corners of a pixel 33′, respectively. Thefirst gate electrode of the first thin film transistor 331′ is connectedto a first scanning line 311′, whereas its two other electrodes areconnected to a first data line 321′ and a pixel electrode 333′respectively. The second gate electrode of the second thin filmtransistor 332′ is connected to a second scanning line 312′, whereas itstwo other electrodes are connected to a second data line 322′ and thepixel electrode 333′ respectively. An in-plane or horizontal electricfield exists between a common electrode 335′ and the pixel electrode333′ to drive liquid crystal molecules to rotate for an opticalresponse. An opaque black matrix 36 which functions as a partition isdisposed at the periphery of the pixel 33′ in order to prevent lightrays from be leaked through the edge of the pixel 33′.

FIG. 8 is a schematic diagram of the circuit layout of the pixel appliedto an active matrix substrate employing the Advanced-Super IPStechnology in accordance with the present invention. In this embodiment,a first thin film transistor 331′ and a second thin film transistor 332′are disposed at two diagonal corners of a pixel 33′, respectively. Thefirst gate electrode of the first thin film transistor 331′ is connectedto a first scanning line 311′, whereas its two other electrodes areconnected to a first data line 321′ and a pixel electrode 333′respectively. The second gate electrode of the second thin filmtransistor 332′ is connected to a second scanning line 312′, whereas itstwo other electrodes are connected to a second data line 322′ and thepixel electrode 333′ respectively. A common electrode 335′ iselectrically connected to its comb-like portion 3351′, thus an in-planeelectric field exists between the comb-like portion 3351′ and the pixelelectrode 333′ to drive liquid crystal molecules to rotate for anoptical response.

The above-described embodiments of the present invention are intended tobe illustrative only. Numerous alternative embodiments may be devised bypersons skilled in the art without departing from the scope of thefollowing claims.

1. A method of driving a pixel unit of a display in which the pixel unithas a first transistor and a second transistor, the method comprising:writing a first voltage via a first data line into the first transistorand a pixel electrode during a first period; writing a second voltagevia a second data line into the second transistor and the pixelelectrode during a second period; obtaining a first waveform oftransmittance from the first voltage returns to a first default, adifference between the first voltage and the second voltage depending onthe first waveform; and obtaining a first area and a second areaenclosed by the first waveform of transmittance and a second waveform oftransmittance, a measure of the first area being equal to a measure ofthe second area.
 2. The method of claim 1, the difference between saidfirst voltage and said second voltage being 0 to 0.5 volts.
 3. Themethod of claim 1, further comprising: writing a third voltage via thefirst data line into the first transistor and the pixel electrode duringa third period; and writing a fourth voltage via the second data lineinto the second transistor and the pixel electrode during a fourthperiod.
 4. The method of claim 3, further comprising: obtaining a thirdwaveform of transmittance resulting from the third voltage returns to asecond default during the third period.
 5. The method of claim 4, adifference between the third voltage and the fourth voltage being 0 to0.5 volts.
 6. The method of claim 4, wherein the difference between thethird voltage and the fourth voltage depends on the third waveform oftransmittance.
 7. The method of claim 6, which further comprising: athird area and a fourth area enclosed by a third waveform oftransmittance and a fourth waveform of transmittance, wherein a measureof the third area is equal to a measure of the fourth area.
 8. Themethod of claim 3, wherein a difference between the second voltage andthe fourth voltage is 0.5 to 15.5 volts.
 9. A driving method for adisplay in which the display has a first pixel unit and a second pixelunit adjacent to each other, each of the first and second pixel unitshaving a first transistor and a second transistor, the driving methodcomprising: controlling the first transistors of the first and secondpixel units and respectively writing a first voltage and a secondvoltage via a first data line into a first pixel electrode and a secondpixel electrode, the first and second pixel electrodes respectivelycoupled to the first transistors of the first and second pixel unitsduring a first vertical scanning period and a second vertical scanningperiod; and controlling the second transistors of the first and secondpixel units and respectively writing a third voltage and a fourthvoltage via a second data line into the first and second pixelelectrodes during the first vertical scanning period and the secondvertical scanning period, each of the first transistors and the secondtransistors of the first and second pixel units being respectivelycontrolled by different control lines.
 10. The driving method of claim9, wherein the first voltage is different than the third voltage. 11.The driving method of claim 10, wherein the first voltage is larger thanthe third voltage.
 12. The driving method of claim 10, wherein adifference between the first voltage and the third voltage is 0 to 0.5volts.
 13. The driving method of claim 9, wherein the second voltage isdifferent than the fourth voltage.
 14. The driving method of claim 13,wherein the second voltage is larger than the fourth voltage.
 15. Thedriving method of claim 13, wherein the difference between the secondvoltage and the fourth voltage is 0 to 0.5 volts.
 16. The driving methodof claim 9, wherein the first transistors of the first and second pixelunits are controlled by a first driver.
 17. The driving method of claim9, wherein the second transistors of the first and second pixel unitsare controlled by a second driver.
 18. The driving method of claim 9,further comprising: recovering a first waveform of transmittanceresulting from the first voltage to a first default transmittance duringthe first vertical scanning period.
 19. The driving method of claim 18,further comprising: enclosing a first area and a second area by thefirst waveform of transmittance and the first default transmittancewherein the measure of the first area is equal to the measure of thesecond area.
 20. The driving method of claim 9, further comprising:recovering a second waveform of transmittance resulting from the secondvoltage to a second default transmittance during the second verticalscanning period.
 21. The driving method of claim 20, further comprisinga step of respectively enclosing a third area and a fourth area by thesecond waveform of transmittance and the second default transmittancewherein the measure of the third area is equal to the measure of thefourth area.
 22. The display of claim 21, wherein the first scanningline and the second scanning line are connected to a first gate drivingdevice.
 23. The display of claim 21, wherein the third scanning line andthe fourth scanning line are connected to a second gate driving device.24. The display of claim 21, wherein the first data line is connected toa first data driving device, and the second data line is connected to asecond data driving device.
 25. The display of claim 21, wherein each ofthe first and second pixel units further comprises a pixel electrodecoupled to the first transistor and the second transistor.
 26. Thedisplay of claim 21, wherein the second scanning line and the thirdscanning line are disposed between the pixel electrodes of the first andsecond pixel units.
 27. A display comprising: a first pixel unit; and asecond pixel unit adjacent to the first pixel unit, each of the firstand second pixel units has a first transistor and a second transistor,the first and second transistors being respectively coupled to a firstdata line and a second data line, each of the first and secondtransistors having gate electrodes, the gate electrodes of the firsttransistors of the first pixel unit and the second pixel unit arerespectively coupled to a first scanning line and a second scanningline, the gate electrodes of the second transistors of the first pixelunit and the second pixel unit are respectively coupled to a thirdscanning line and a fourth scanning line, the second scanning line beingadjacent to the third scanning line.